In a typical cellular radio system, also referred to as a wireless communication system, user equipments, also known as mobile terminals and/or wireless terminals, communicate via a Radio Access Network (RAN) to one or more core networks. The user equipments may be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability, and thus may be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with the radio access network. The user equipment may also be referred to as a communication device, a terminal or a UE.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Base station (BS), which in some networks is also called Radio Base Station (RBS), “eNB”, “eNodeB”, “NodeB” or “B node” and which in this patent also is referred to as a base station. A cell is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g. by landlines or radio link, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
FIG. 1 is a schematic block diagram illustrating the architecture of a Radio Access Network (RAN) 101.
The UE 102 is the mobile terminal by which a subscriber can access services offered by the operator's Core Network (CN) 103.
The RAN 101 is the part of the network that is responsible for the radio transmission and control of the radio connection.
The Radio Network Subsystem (RNS) 104 controls a number of Base Stations (BS) 105 in the radio access network.
The Radio Network Controller (RNC) 106 controls radio resources and radio connectivity within a set of cells.
The BS 105 handles the radio transmission and reception within one or more cells.
A cell covers a geographical area. The radio coverage in a cell is provided by base station equipment at the base station site. Each cell is identified by a unique identity, which is broadcast in the cell. There may be more than one cell covering the same geographical area.
A radio link is a representation of the communication between a UE 102 and one cell in the RAN 101.
The Iub/Iur interfaces are interfaces connecting the different nodes in the RAN 101. User data is transported on so-called transport bearers on these interfaces. Dependant on the transport network used, these transport bearers could e.g. be mapped to AAL2 connections (in case of an ATM based transport network) or UDP connections (in case of an IP based transport network).
In 3GPP Release 99, the retransmission scheme of the dedicated channels (DCH) is part of the radio link control (RLC) protocol layer, which terminates in the UE 102 and the RNC 106. When High Speed Downlink Packet Access (HSDPA) was introduced, some control functions were relocated from the RNC to the base station. These include fast retransmissions in the media access control (MAC) layer from the base station when transmissions fail. Also an enhanced uplink (EUL) has been introduced in WCDMA with similar retransmission mechanisms in MAC. The collective term High Speed Packet Access (HSPA) is often used for the combination of HSDPA and EUL.
A retransmission scheme with both error correction and error detection is referred to as hybrid ARQ (Automatic Repeat-reQuest). Error corrections are enabled by combining information from both the first transmission of a data block and from subsequent retransmissions of the same data block. Furthermore, it is also possible to consider transmitting additional coded bits instead of repeating the same data block during a retransmission. To allow time for processing and signaling, several data blocks are handled in parallel. While data block i is processed and decoding information is fed back to the transmitter.
Random access in UTRAN is based upon slotted ALOHA. UE's in idle state monitors the system information of a base station within range to inform itself about candidate base stations in the service area etc. When a UE needs access to services, it sends a request over the random access channels (RACH) via the most suitable base station, typically the one with the most favorable radio conditions. Since the uplink propagation is only approximately known, the UE gradually increases the transmission power of a preamble until either it has been acknowledged via the Acquisition channel AICH, or the maximum number of attempts has been reached. Upon acknowledgement, the RACH message is sent. Reference is her made to 3GPP TS 25.214, Physical layer procedures (FDD) and 3GPP TS 25.211, Physical channels and mapping of transport channels onto physical channels (FDD). After admission control, the RNC initiates the connection via the most suitable base station if there are available resources. Uplink coverage is thus a necessity in order to successfully complete random access.
The UE can start the random-access transmission at the beginning of a number of well-defined time intervals, denoted access slots. There are 15 access slots per two radio frames and they are spaced 5120 chips apart, see FIG. 2. The timing of the access slots and the acquisition indication is described later. Information on what access slots that are available for random-access transmission is given by higher layers.
The structure of the random-access transmission (RACH Transmission) is shown in FIG. 3. The random-access transmission consists of one or several preambles of length 4096 chips and a message of length 10 ms or 20 ms.
Each preamble is of length 4096 chips and consists of 256 repetitions of a signature of length 16 chips. There are a maximum of 16 available signatures, see 3GPP TS 25.213: “Spreading and modulation (FDD)” for more details.
Before the physical random-access procedure can be initiated, Layer 1 shall receive the following information from the higher layers (RRC):                The preamble scrambling code.        The message length in time, either 10 or 20 ms.        The AICH_Transmission_Timing parameter [0 or 1].        The set of available signatures and the set of available RACH sub-channels for each Access Service Class (ASC).        The power-ramping factor Power Ramp Step [integer>0].        The parameter Preamble Retrans Max [integer>0].        The initial preamble power Preamble_Initial_Power.        The Power offset P p-m=Pmessage-control−Ppreamble, measured in dB, between the power of the last transmitted preamble and the control part of the random-access message.        The set of Transport Format parameters.        
The physical random-access procedure shall be performed as follows (excluding signalling to higher layers).    1. The available uplink access slots are derived.    2. Randomly select a signature from the set of available signatures within the given ASC.    3. Set the Preamble Retransmission Counter to Preamble Retrans Max.    4. Calculate initial preamble power, considering the limited UE power.    5. Transmit a preamble using the selected uplink access slot, signature, and preamble transmission power.    6. If no positive or negative acquisition indicator (AI≠+1 nor −1) corresponding to the selected signature is detected in the downlink access slot corresponding to the selected uplink access slot:            6.1. Select the next available access slot in the set of available RACH sub-channels within the given ASC.        6.2. Randomly select a new signature from the set of available signatures within the given ASC.        6.3. Increase preamble power by □P0=Power Ramp Step [dB].        6.4. Decrease the Preamble Retransmission Counter by one.        6.5. If the Preamble Retransmission Counter>0 then repeat from step 5. Otherwise exit the physical random access procedure.        6.6. If a negative acquisition indicator is detected exit the physical random access procedure.            7. Transmit the random access message three or four uplink access slots after the uplink access slot of the last transmitted preamble depending on the AICH transmission timing parameter. Transmission control part power should be P p-m [dB] higher than the power of the last transmitted preamble.
The preamble detection mechanism in the Node B is based on received preamble correlation (can be interpreted as received energy) relative a preamble, which is configured from the RNC to the RBS over NBAP, see 3GPP TS 25.433, NBAP, UTRAN Iub interface NBAP signaling for further information. A too low threshold would mistakenly trigger preambles from thermal noise, and similarly, a too high threshold will trigger preambles at very high power levels, or miss preambles all together. The threshold may be set considering the worst case uplink load situation.
A RACH sub-channel defines a sub-set of the total set of uplink access slots. There are a total of 12 RACH sub-channels, essentially using one twelfth of the uplink slots each. FIG. 4 illustrates the access slots associated with the different sub-channels.
The set of allowed sub-channels are signaled from higher layers, depending on the UE access service class, see 3GPP TS 25.331, “Radio Resource Control (RRC)” for further information.
As described in 3GPP TS 25.211, “Physical channels and mapping of transport channels onto physical channels (FDD)” Section 7.3, the downlink AICH is divided into downlink access slots, and each access slot is of length 5120 chips. AICH access slots #0 starts the same time as P-CCPCH frames with (SFN modulo 2)=0. Similarly, the uplink PRACH is divided into uplink access slots, each access slot is of length 5120 chips. Uplink access slot number n is transmitted from the UE τp-a chips prior to the reception of downlink access slot number n, n=0, 1, . . . , 14.
Transmission of downlink acquisition indicators may only start at the beginning of a downlink access slot. Similarly, transmission of uplink RACH preambles and RACH message parts may only start at the beginning of an uplink access slot.
The PRACH/AICH timing relation is shown in FIG. 5.
The preamble-to-preamble distance τp-p shall be larger than or equal to the minimum preamble-to-preamble distance
τp-p,min, i.e. τp-p≧τp-p,min.
In addition to τp-p,min, the preamble-to-AI distance τp-a and preamble-to-message distance τp-m are defined as follows:                when AICH_Transmission_Timing is set to 0, then        τp-p,min=15360 chips (3 access slots)        τp-a=7680 chips (1.5 access slots)        τp-m=15360 chips (3 access slots)        when AICH_Transmission_Timing is set to 1, then        τp-p,min=20480 chips (4 access slots)        τp-a=12800 chips (2 access slots)        τp-m=20480 chips (4 access slots)        
The parameter AICH_Transmission_Timing is signalled by higher layers.
FIG. 6 illustrates the available preambles for the first RACH preamble transmission relative the time of downlink access slot #0.
If the initial preamble transmission is not acknowledged over AICH, the UE selects a new access slots among the access slots associated to the allowed sub-channels, considering that the next preamble is at least either 3 or 4 access slots later, depending on the AICH_Transmission_Timing parameter.
For example, if the UE is allowed to use sub-channels 0-3, and τp-p,min corresponds to 3 access slots, then    If the UE selected access slot #0 for the first preamble transmission, a possible preamble retransmission can take place in access slot #3.    If the UE selected access slot #1 for the first preamble transmission, a possible preamble retransmission can take place in access slot #12 at earliest.
In the 3GPP release 99, the RNC controls resources and user mobility. Resource control in this framework means admission control, congestion control, channel switching (roughly changing the data rate of a connection). Furthermore, a dedicated connection is carried over a dedicated channel DCH, which is realized as a DPCCH (Dedicated Physical Control Channel) and a DPDCH (Dedicated Physical Data Channel).
In the evolved 3G standards, the trend is to decentralize decision making, and in particular the control over the short term data rate of the user connection. The uplink data is then allocated to E-DCH, which is realized as the triplet: a DPCCH, which is continuous, an E-DPCCH for data control and an E-DPDCH for data. The two latter are only transmitted when there is uplink data to send. Hence the Node B uplink scheduler determines which transport formats each user can use over E-DPDCH. The RNC is however still responsible for admission control.
A data block is sent by the UE to the NodeB during a transmission time interval (TTI). For efficiency reasons, the received data blocks at the receiver are processed in parallel at M parallel processors taking turn to process data. While data block i is processed and decoding information is fed back to the transmitter, the receiver starts processing data blocks i, i+1, . . . etc. By the time when the receiver processor 1 has decoded the data block and fed back the decoding result, it is ready for processing either a retransmission of information related to the recently processed data or a new data block. By combining information both from the original data block and the retransmission, it is possible to correct errors in the reception. A retransmission scheme with both error correction and error detection is referred to hybrid ARQ. Therefore, the M processors are often referred to as HARQ processes, each handling a data block received in a TTI. FIG. 7 depicts parallel HARQ processors for M=8.
In the WCDMA uplink, there is a trade-off between coverage and enabled peak rates. This is even more emphasized with enhanced uplink, which supports higher bit rates than ordinary dedicated channels. The uplink resources are limited by the rise over thermal (RoT) that the cell can tolerate. The RoT limit is either motivated by coverage requirements or power control stability requirements. When only one user is connected in the cell, both power control stability and coverage are minor issues, since the uplink interference is likely to be dominated by the power generated by this user. In such a case it is tempting to allow a high RoT in order to allow high received signal relative interference powers, Ec/Io, which enables the use of high uplink bit rates. Conversely, in order to use the high uplink bit rates, the user connections have to provide high Ec/Io, which implies high RoT.
In order to orthogonalize the uplink user transmissions to a greater extent, it can be relevant to separate the user data transmissions in time, and employ a TDM (time division multiplex) scheme. It is possible to allocate grants to a user that is only valid for specified HARQ processes. This fact can be exploited to enable TDM for EUL. Furthermore, it allows retransmissions without interfering with other users, since retransmissions hit the same HARQ process as the original transmission. FIG. 8 provides some example resource allocations in a TDM setting.
In CELL_FACH, the UTRAN may redirect the UE to another frequency, see 3GPP TS 25.331, “Radio Resource Control (RRC)” for further information.
When in the CELL_FACH state, the UE autonomously selects carrier (in 3GPP specifications, referred to as cell reselection) and signals the selected carrier according to a specified “cell update” procedure, see 3GPP TS 25.331, “Radio Resource Control (RRC)” for further information, and FIG. 9 which illustrates UTRA RRC Connected mode cell reselection for URA_PCH, CELL_PCH, and CELL_FACH.
The cell reselection is essentially based on measurements of downlink signal quality of the common pilot channel (CPICH), which is broadcasted in each cell with a constant transmit power. More specifically, there are two options for quality metrics:    Energy per chip divided by the total received non-orthogonal interference power (Ec/NO) of the common pilot channel (CPICH).    Received signal code power (RSCP, i.e. signal strength) of the CPICH.
Which metric to employ is decided by the network and signalled on the broadcast channel (BCH).
The cell (re)-selection applies both to cells on the same carrier frequency, but also on other carrier frequencies.
As specified in 3GPP TS 25.304, “User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode”, the ranking of each cell is given by:Rs=Qmeas,s+Qhysts for the current cell andRn=Qmeas,n−Qoffsets,n for the neighbouring intra/inter-frequency cells.
There are also other parameters involved, e.g. for priorities between cell layers which is useful for hierarchical cell structures, that are omitted here for the sake of clarity. See further 3GPP TS 25.304, “User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode”.
Observe that Qoffsets,n is an offset for the pair of cells and Qhysts. is a hysteresis margin employed for the current cell.
Dual-Carrier High-Speed Downlink Packet Access (DC-HSDPA, also known as Dual-Cell HSDPA) was introduced within the 3rd Generation Partnership Project (3GPP) Rel-8. DC-HSDPA enables reception of data from two cells simultaneously, transmitted on two adjacent carriers in the same base station and sector, to individual terminals (or user equipments, UEs). The concept of DC-HSDPA is in 3GPP Rel-10, being extended to 4 downlink carrier frequencies (known as 4C-HSDPA).
To complement DC-HSDPA, in 3GPP Rel-9, Dual-Carrier High-Speed Uplink Packet Access (DC-HSUPA) was also introduced. DC-HSUPA enables an individual terminal to transmit data on two adjacent carrier frequencies simultaneously to the radio access network. DC-HSUPA according to 3GPP Rel-9 is in essence an aggregation of legacy (Rel-8, single-carrier) HSUPA.
Although the additional spectrum bandwidth associated with multi-carrier operation does not increase “spectral efficiency” (maximum achievable throughput per cell per Hz [bps/cell/Hz]), the experienced user data rates are increased significantly. In particular, for bursty packet data traffic at low and moderate load, the data rate is proportional to the number of carriers exploited. Moreover, power inefficient higher order modulation schemes can be avoided (which is especially important in the uplink) and the practical as well as theoretical peak data rate of the system are naturally increased.
In the discussion below the focus will be on the case of DC-HSUPA and 4C-HSDPA using contiguous carrier frequencies. However, all concepts are readily extendable to Multi-Carrier HSUPA (MC-HSUPA) operation over more than two uplink carriers, and system configurations wherein the carrier frequencies employed for the respective link direction are non-contiguous (e.g., in located in different frequency bands).
Handover and radio access bearer admission control is presumed to be conducted in the Radio Network Controller (RNC) based on measurements of path loss etc on a primary carrier (alternatively referred to as an anchor carrier). Notice though, that in case of a distributed RAN architecture where Node-B and RNC functionality as defined in 3GPP specifications are collocated in a base station, the base station would naturally handle also these functionalities. In a DC-HSUPA capable Node-B, the other carrier, which is referred to as a secondary carrier, is assumed to be configured by the RNC for a given DC-HSUPA capable UE and then scheduled and activated by Node-B whenever feasible and useful (with the standard objective function to maximize the supported traffic volumes, or aggregate system throughput, subject to fairness criteria and quality of service constraints, such as minimum bit rate or maximum latency requirements). A primary carrier, on the other hand, may not be temporarily deactivated by the Node-B: to deactivate a certain primary carrier for a connection, the connection is either released, or an inter-frequency handover is performed (in which case another carrier will become the primary carrier).
For each user connected in DC-HSUPA mode, the serving Node-B hence controls whether or not a secondary carrier is activated, and a separate grant is selected for each activated carrier.
Furthermore, if a secondary carrier is activated by Node-B, it is assumed that the Dedicated Physical Control Channel (DPCCH), which includes a sequence of pilot bits, is transmitted on that carrier, and the Node-B hence tries to detect this signal.
In a future system, one can envisage multi-carrier operations in the CELL_FACH state. A natural extension would then be to introduce a Node-B controlled carrier selection of the uplink transmissions and this will be presumed in certain embodiments of this invention.
Problems with existing solutions are summarized below.
Cells operating at high RoT will have limited coverage, and it might be impossible to successfully complete random access from some parts of the service areas. The RACH preamble will not be detected by the system at these high RoT when sent from parts of the service area. Furthermore, the gradual power increase may generate significant interference, which could have a negative impact on the data rate of the active user(s). Neither is it possible to lower the preamble because this will lead to many erroneous preamble detections from only thermal noise, which will give unnecessary Iub transmissions of subsequent RACH messages. Further, this will in many cases result in subsequent RACH transmissions at too low power levels, which will not be decoded correctly.
Moreover, the short time between a received preamble and when an acquisition indicator is expected means that it is very limited time for processing before it has to be determined whether a preamble was sent at a sufficient power level.
In the future operation at significantly higher RoT than today is anticipated in order to support high data throughput and many connected users in the cellular networks. The present RACH performance will then become a bottle-neck, making it impossible for terminals doing random access close to the cell edge to be detected by the base station. This situation will become even more troublesome when advanced interference suppressing receivers such as the G-rake+ are introduced. These receivers allow a higher RoT over the air-interface, making RACH performance even more critical. The solution to equip the RACH channel with these receivers as well, is only expected to provide a partial solution to the problem. It is e.g. non-trivial to determine the load after that IS gains have materialized for example for G-rake+ receivers allocated to RACH.
As a consequence another specific problem, associated with the present invention, is that signalling of RoT measured after advanced receivers like G-rake+ is not supported by the WCDMA standard. This limitation is at hand in the NBAP and RNSAP interfaces.
Still another specific problem is that the UE does base its carrier selection exclusively on the interference situation in the downlink—even in cases where e.g. measurements of uplink load would be available in the cellular system.